I think the reason this card was graded by BGS is because a reader wrote in and Beckett questioned Topps and received a response that cards stuck together and it was inserted into packs.

Without that confirmation from Topps they might not have graded it because like you said it does say Not Valid Without Sticker.

Perhaps BGS needs to rethink on what they grade or how they grade errors.

We have seen BGS grade trimmed cards, fake cards that have replaced patches, or even fake sig cards like the ones from buybacks in bowman origins. They as the "authenticator", or "experts" are running a different business than topps. Topps produce cards, they have no reason to deal with what beckett does I believe. If Beckett has to contact Topps for every little thing, what does that say about thier business, thier expertise goes out the door doesn't it? So as professionals, they have information that is released and goes by what is presented to them.